1. Field of the Invention
The present invention relates to passive ring resonators utilizing counter-rotational beams of electromagnetic energy to sense the rate of rotation of such resonators through the shift in resonant frequency induced by rotation about an inertial axis.
2. Description of the Prior Art
Ring resonators of the prior art normally include an active lasing medium disposed therewithin to direct light waves emitted in the active medium in opposite directions around a closed loop, normally planar path. The counter-rotating light beams form an effective standing wave pattern and oscillate at the same frequency when their respective path lengths are equal, but at different frequencies when the path lengths are unequal because of rotation of the path loop about an inertial axis generally perpendicular to the propagation plane of the light waves. Through the detection of this frequency difference, the active ring laser of the prior art is enabled to sense the rate of rotation of its path loops and thus has found wide acceptance in navigation and guidance applications. The prior art ring laser rate of turn device is well suited to use in modern navigation and guidance systems employing high speed data processing because it can be turned on very rapidly and provides virtually instantaneous response to input rotation rate changes. Typical practical forms of such ring laser devices are disclosed in the following U.S. Pat. Nos. assigned to Sperry Corporation:
C. C. Wang--3,382,758 for a "Ring Laser Having Frequency Offsetting Means Inside Optical Path", issued May 14, 1968, PA1 W. M. Macek--3,382,759 for a "Ring Laser Biased by Zeeman Frequency Offset Effect for Sensing Slow Rotations", issued May 14, 1968, PA1 W. M. Macek--3,382,760 for a "Coherent Light Frequency Difference Sensor", issued May 14, 1968, PA1 W. M. Macek--3,508,831 for a "Ring Laser Having Minimized Frequency Locking Characteristic", issued Apr. 28, 1970, and PA1 W. M. Macek--3,480,878 for a "Ring Laser With Means for Reducing Coupling to Backscattered Waves", issued Nov. 25, 1969.
The foregoing patents testify to the fact that such active ring rate of turn laser devices, i.e., those having an internal gain mechanism within the measuring ring, demonstrate undesired mode locking phenomena proximate the zero rate of rotation situation. In this condition, the measured output frequency undesirably remains constant over a small range of rates of rotation including and on each side of zero rate of turn. Accordingly, measures must be taken to overcome this effect, such as by the use of mirrors in the laser ring resonator designed to minimize back scatter of light and by the use of rate biasing techniques which induce a significant difference in frequency between the counter-rotating beams, desirably preventing mode locking at the zero rate of turn condition.
Passive ring rate of turn measurement devices, i.e., those containing no active gain element within the measuring ring or loop, have been recently demonstrated, as reported by S. Ezekiel and S. R. Balsamo, "Passive Ring Resonator Gyroscope", Applied Physics Letters, Vol. 30, page 478 et seq. (1977). Related material is presented in the S. Ezekiel U.S. Pat. No. 4,135,822, issued Jan. 23, 1979 for a "Laser Gyroscope". Since the passive ring device does not include a gain medium within its passive ring, some of the problems associated with the presence of such a gain medium are avoided. However, such prior art passive ring resonator devices have used detection schemes wherein each of the counter-rotating beams is measured and, by means of one or more closed loop feed back schemes, the frequency characterizing at least one of the counter-rotating light beams is adjusted to a predetermined value. There is no attempt to generate a useful rotation rate output signal until this adjustment is accomplished by the feed back apparatus. While fast acting components for such feed back servo schemes are available, the noise introduced in practice by the use of such components increases with increasing speed of response, introducing a significant source of error into the output of the rate of turn system. Accordingly, such prior art passive resonator rate of turn devices do not match the capabilities of high speed data processor systems normally employed in precision navigation.
In order to overcome some of the problems of prior art passive ring rate of turn measurement apparatus, the device of the C. D. Lustig U.S. Pat. No. 4,274,742 issued June 23, 1981 for a "Passive Ring Laser Rate of Turn Indicator" and assigned to Sperry Corporation, was proposed. In the Lustig arrangement, a ring laser system is provided having means forming a passive resonator for the propagation of clockwise and counter-clockwise beams of electromagnetic energy. The clockwise and counter-clockwise resonant frequencies are oscillated between respective first and second distinct values. Detector means are provided for producing signals with amplitudes indicative of the amplitudes of the first and second beams. A further means is coupled with the detector means for producing a rotation rate signal proportional to the algebraic combination of the signals indicative of the amplitudes of the first and second beams. By dispensing with closed loop feed back arrangements to provide an output signal indicative of rotation rate, the invention provides rapid response to changes in the rotation rate of the passive resonator and is thus adapted for use with high speed data processors.
In the aforementioned Ezekial and Lustig devices, the resonant optical path is stabilized at the resonance frequency by mounting one of the mirrors on an electrically activated piezoelectric element. The resonant ring path length is adjusted by varying a d.c. bias voltage applied across the piezoelectric element; the path length itself is also modulated by applying an alternating voltage across the element. The depth of the latter modulation is great enough to span the half-power points of the resonance curve so as to maximize the sensitivity of the stabilization feed back loop.
The laser light flowing through the resonant path is detected by a photodetector coupled in the stabilization loop. If the path length is off resonance, no light is transmitted to the photodetector; at resonance, maximum light is transmitted. If the bias voltage is such that the modulation is symmetric about the resonance point, the photodetector output at the modulation frequency is zero. When slightly off resonance, the amplitude of the photosignal depends on the magnitude of the departure of the bias from the resonance condition and its sign depends on the associated directional sense of departure. The feed back control uses the photodetector signal to adjust the bias to bring the optical path back to its resonance point.
One of the problems of the prior art solved according to the present invention relates to the band width of the feed back loop and the noise frequency spectrum. Microphonic conditions cause high finesse optical resonators to show noise fluctuations from resonance; these fluctuation frequencies reach up into the high audio range. For the stabilization loop to respond to such high frequency fluctuations, the modulation signal must be high compared to the noise frequencies.
The conventional piezoelectric path length modulator on which one of the mirrors is customarily mounted is massive and cannot be driven at frequencies higher than about a few hundred Hertz. At higher frequencies, mechanical inertia severely distorts the detected line shape. At frequencies in excess of a few kilo-Hertz, the mirror-driver system completely fails to respond to the input modulation signal.
Fluctuations in the resonant optical path or line jitter not only cause problems in the stabilization loop but, under certain circumstances, they add excessive noise to the rate signal output itself. It is observed experimentally that the two resonances of the counter-rotating light beams occur at optical path lengths which often differ by large amounts compared to the line displacements caused by rotation. In a high finesse ring with confocal mirrors, slight differences in the direction or point of entry of the light beams into the resonant path can lead to such large errors. This resonance separation will appear at the rate of turn output as a high rate bias. These fluctations cause the optical path length to sweep randomly between the two resonances, and this effect superimposes a large noise signal on the difference in the voltage output by the two photodetectors.